Nonaqueous phase organic liquids are widespread contaminants of subsurface soils and groundwater. From a human health perspective, these organic liquids are of particular concern because they tend to represent long-term sources of pollution and create diverse and persistent pathways for eventual human exposure. The work proposed in this project is directed towards the investigation of processes which influence the entrapment, persistence, transport, bioavailability, and toxicity of organic liquid components in the natural subsurface environment. It represents a fundamental research program which builds upon previous and current NIEHS-funded projects and integrates laboratory and mathematical modeling investigations. Subproject A focuses on the behavior of benzene, toluene, xylene (BTX) and chlorinated solvent compounds. Its specific aims relate to the investigation of the influence of physical and chemical heterogeneities on organic liquid entrapment and mass transfer, and the quantification of the influence of multiple, coupled processes on contaminant fate and transport. Special attention will be directed to the elucidation of microbial processes through close collaboration with Project 1 investigators. Thus, the work proposed is designed to logically extend the knowledge gained in previous NIEHS- funded projects to consider the interaction of multiple processes, the influence of chemical and physical heterogeneities on these processes, and the behavior and fate of complex NAPL mixtures. Although directed primarily at the investigation of naturally occurring attenuation processes, the knowledge gained from these studies will have immediate applications to the design of more effective remediation strategies.